1. Field of the Invention
The present invention relates to an image communication system which reads a document image, stores the read image data into a memory and transmits the image data stored in the memory.
2. Related Background Art
A facsimile machine has been known in the art as such a system.
A facsimile machine having a memory store transmission mode in which an image read from a document is temporarily stored into a memory, a facsimile protocol is executed to a destination machine and the image is transmitted, has also been known in the art.
In a normal transmission mode (document transmission mode) in which reading of the document and transmission are simultaneously conducted, the document may be read in a pre-protocol of communication with a resolution power (for example, 3.85 lines/mm, 7.7 lines/mm or 15.4 lines/mm) which is compatible with a receiving capability of the destination machine.
However, in the memory transmission mode, the reading of the document has already been finished before the execution of the pre-protocol. Thus, it is not always true that the transmission is done at the resolution power of the reading, and if the resolution powers do not match, the transmitting machine must take out some main scan lines of the read image data.
However, the take-out (or thinning) of the main scan lines causes a two-dimensional variation of binary data and deteriorates an image quality.
For example, when a conversion from 15.4 lines/mm to 7.7 lines/mm is done, it means that non-contiguous main scan lines read from a narrow area along a sub-scan direction are enlarged by a factor of two in a print mode. Thus, the image quality is lower than that attained by reading the document at 7.7 lines/mm.
Particularly, in a half-tone image, half-tone is pretendedly expressed by an average density of binary pixels. Accordingly, if the main scan lines are taken out, the image quality is very poor.
Where a size of document read and a size of a record sheet at the receiving station are different, the same problem as that in the mismatch of the resolution power takes place.
When a destination station and the memory transmission mode are designated from a console panel, an image is read by a reader at a unity magnification (real-size), and the image data is encoded and the data of the entire document is temporarily stored in a memory. Then, the designated destination station is called and the pre-protocol is executed.
When the image data is transmitted to the destination station, the image data stored in the memory is temporarily decoded and it is reproduced as an original image in a transmission line memory, and it is re-encoded in accordance with a code mode permitted by the destination receiving machine and sent out.
If a width of the document is larger than a width of the record sheet of the destination receiving machine, it is reduced at the transmitting station as described above. This is done in the re-encode mode by the random take-out of pixels in the main scan direction and the random take-out of lines in the sub-scan direction.
In the case of mismatching of sizes, the binary image stored in the memory is to be reduced in size. As a result, the image is distored and fine lines are lost, and the image quality is lowered.
Particularly when the half-tone image represented by binary data is reduced in size, the distribution of binary pixels which represent half-tone pretendedly becomes ununiform, and moire fringe, longitudinal fringe or lateral fringe appears. Thus, the degradation of the image quality due to the size reduction is more remarkable than that for a toneless image.
Patent applications relating to a facsimile machine having a memory transmission mode are U.S. Pat. Nos. 4,796,092, 4,814,890, 4,772,955 and 4,845,569.
However, no proposal to solve the above problems has been made.